Ethernaut is a security-focused game that presents levels as Solidity smart contracts that you, the player, need to hack. Through the game you learn about Ethereum smart contract vulnerabilities and how to find exploits and bugs in smart contract code. Over time, the challenges get more and more tricky to solve.
In this video, we attempt Level 1 of Ethernaut - Fallback - where we steal ETH from a smart contract by sending it ETH :)